Audio mixing is a process by which a multitude of sound sources are combined into one or more signals. Signals from the sources might be live or recorded and could be generated from different musical instruments, vocals, orchestra sections, announcers or crowd noise. A source signal's amplitude, frequency content, dynamics and panoramic position can be manipulated during mixing to add effects, such as reverberation, for example.
Recording studios make use of multiple microphones to record a single event. Microphones may be placed above and below drums, as well as in front and in back of drums, for example. The same may be true of other musical instruments as well as for vocalists. Resulting electrical signals from the microphones are then combined in artistic, as well as technical methods, so as to produce an elevated mix that is more appealing to listeners.
In one example, a microphone may be placed near a North side of the drum membrane, and another may be placed near a South side of the drum membrane. When the drum makes a sound, the top microphone will receive sound waves of a reference phase, and the bottom microphone will receive sound waves 180° out of phase from the reference phase. This follows because when the diaphragm of the drum is swinging toward the North side, the diaphragm is simultaneously swinging away from the South side. Thus, a signal received by the top microphone will be nearly 180° out of phase with a signal received by the bottom microphone. If the signals from the two microphones are added together, the signals will largely cancel each other out, which is not acceptable. Thus, one common solution is to invert the signal from the bottom microphone by 180° before the signal is added to (or “mixed with”) the signal from the top microphone. Alternatively, the signal from the bottom signal may be delayed with respect to the signal of the top microphone before the signals are combined. If there were also microphones placed near an East and/or West side of the drum, it can become more difficult to combine all of the signals so that the signals do not cancel portions of each other out.